Ethylene copolymers are molded by various molding methods and used in many fields. The properties required for the ethylene copolymers differ from each other according to the molding methods and the uses of the copolymers. For example, in the production of an inflation film by a high-speed molding method, an ethylene copolymer having high melt tension for its molecular weight must be selected in order to stably perform high-speed molding free from occurrence of bubble swing or bubble break. The similar properties are required to prevent sag or break in a blow molding method or to lower reduction of width to the minimum in a T-die molding method.
In Japanese Patent Laid-Open Publication No. 90810/1981 or No. 106806/1985, a method of increasing melt tension or swell ratio (die swell ratio) of ethylene polymers obtained by the use of Ziegler catalysts, particularly titanium catalysts, to improve moldability of the polymers is reported. In general, the ethylene polymers obtained by the use of the titanium catalysts, particularly low-density ethylene copolymers, however, have wide composition distribution and contain components which cause tackiness when the polymers are used as molded products such as films. Therefore, decrease of the components causing tackiness has been demanded.
Of the ethylene polymers produced by the use of Ziegler catalysts, those obtained by the use of chromium catalysts have relatively high melt tension, but further improvement in the heat stability has been demanded.
A great number of ethylene copolymers obtained by the use of olefin polymerization catalysts containing transition metal metallocene compounds have high melt tension and excellent heat stability, so that they are expected as copolymers filling the above demands. In the ethylene copolymers obtained by the use of the metallocene catalysts, however, the melt tension (MT) is generally proportional to the flow activation energy (Ea).
Polymers having high melt tension show excellent moldability because the bubble stability is good as mentioned above, but they show high flow activation energy (Ea), and this means that the molding conditions thereof have great dependence on the temperature. Therefore, if the molding conditions are not controlled very strictly and uniformly, the resulting molded products suffer unevenness. For example, films may have low transparency.
When the flow activation energy (Ea) is low, occurrence of unevenness in the molded products can be inhibited, but because of low melt tension, unstable bubble is produced and hence moldability is lowered.